Bulk ore tailings from the Macraes gold mine gold
extraction process are stored in a large (ca 0.6 km2, 80 m deep)
mixed tailings impoundment whose pH is maintained near 8 by natural rock
neutralizing capacity. The mixed tailings will be stored in this impoundment in
perpetuity after mining ceases. Confidence in the long term environmental
stability of these tailings can be gained from examination of mineralogically
and chemically similar geological situations in the immediate vicinity. A
sequence, typically about 5 m thick, of sands and gravels derived from the
Macraes mineralized zone 12 000 to 28 000 years ago contains rounded detrital
sulphide mineral grains which are unoxidized despite close proximity to the
surface and occasional incursion of oxygenated waters. Saturating water pH is
currently 7-8. Sands with 0.2 to 0.8 wt % organic carbon host sulphate-reducing
bacteria (SRB), and localized cementation by authigenic framboidal pyrite has
occurred. Comparison of the 28 000 year-old sediments with the modem tailings
suggests that similar chemistry will prevail, possibly with deposition of
authigenic pyrite in the tailings, in the long term. As long as the present
slightly anoxic and circumneutral pH environmental conditions are maintained in
the mixed tailings dam, sulphide decomposition and acidification are unlikely.
These sediments provide a plausible geological analogue for long-term benign
storage of the mixed tailings impoundment.